This CL reverts the changes made in golang/go#11299.
Allowing creation of multiple contexts break the case of multiple
third party packages depending on the al bindings. These packages
need to make sure that there is a valid current context or initiate
and make a context current. Packages racing to create and make a
context current is what we would like to avoid.
Therefore, al bindings will support only a single context that is
initiated during OpenDevice.
Fixesgolang/go#11385.
Change-Id: I662f70e49d12833a545005cf0724cc21f67bea09
Reviewed-on: https://go-review.googlesource.com/12001
Reviewed-by: Nigel Tao <nigeltao@golang.org>
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